1. Field of the Invention
The invention relates to the preparation and use of rubber powders or pellets based on rubber-latex emulsions and containing silicatic and/or oxidic fillers, by precipitation from an aqueous phase.
2. Discussion of the Background
A wide variety of publications and patents (WO PCT/EP 99/01970; WO PCT/EP 99/0171; EP 99 117 178.6; U. Görl, K.-H. Nordsiek, Kautsch. Gummi Kunstst. 51 (1998) 200; Rubber World 3/01 and 4/01; and U. Görl, M. Schmitt, O. Skibba, Gummi Fasern Kunstst. 54 (2001) 532) have appeared relating to the purpose and benefits of the use of rubber powder and potential processes for its preparation.
The interest in pulverulent rubber/filler masterbatches can be explained as a necessary result of the processing technology used in the rubber industry (E. T. Italiander, Gummi Fasern Kunstst. 50 (1997) 456), where rubber mixtures are prepared with high costs for energy, time, and personnel. One prime reason for this is that the rubber raw material is in bale form, and its processing requires the mechanical incorporation and dispersion of large amounts of active fillers (industrial carbon blacks, silicas, and naturally occurring fillers) into the rubber phase.
In industry, the mechanical kneading process generally takes place batchwise in large internal mixers or on roll mills, generally in a process with two or more stages.
To simplify these complicated steps in a process (U. Görl, K.-H. Nordsiek, Kautsch. Gummi Kunstst. 51 (1998) 200; Rubber World 3/01 and 4/01; and U. Görl, M. Schmitt, O. Skibba, Gummi Fasern Kunstst. 54 (2001) 532) or even provide an opportunity for developing and introducing new continuous processes (E. T. Italiander, Gummi Fasern Kunstst. 50 (1997) 456; R. Uphus, O. Skibba, R.-H. Schuster, U. Görl, Kautsch, Gummi Kunstst. 53 (2000) 279), rubber powder technology (Delphi-Report ‘Künftige Herstellungsverfahren in der Gummiindustrie’ (Future production processes in the rubber industry), Rubber J. 154 (1972) 20; and Kautsch. Gummi Kunstst. 26 (1973) 127) has long been regarded as one of the most suitable options. It combines the necessity for prior incorporation of a filler with a specific form of presentation, which is a powder or, respectively, pellets capable of free flow and therefore capable of automatic metering and conveying.
Whatever the type of filler (carbon blacks, silicas, naturally occurring fillers, etc.), the rubber powders/pellets are prepared by precipitation from a mixture of a filler suspension in water and a rubber-latex emulsion, by using a Brönsted and/or Lewis acid to lower the pH (WO PCT/EP 99/01970; WO PCT/EP 99/0171; EP 99 117 178.6; U. Görl, K.-H. Nordsiek, Kautsch. Gummi Kunstst. 51 (1998) 200; Rubber World 3/01 and 4/01; and U. Görl, M. Schmitt, O. Skibba, Gummi Fasern Kunstst. 54 (2001) 532). The preparation may be carried out batchwise or continuously (EP 00 104 112.8).
The formation of rubber powder/pellets, composed of polymer and filler, via addition of acid can be thought of as adsorption of the rubber on the filler. Here, the interaction between the filler surface and the rubber chains is important. The strength of this interaction is in turn determined by the polarity differences between the two starting materials: rubber and filler.
Industrial carbon blacks and most familiar grades of rubber are non-polar, i.e. have a high level of mutual interaction, the result being that rubber powder/pellets composed of these two components are generally capable of easy preparation from the abovementioned precipitation process without using any other additives or measures.
In contrast, silicatic and oxidic fillers are polar. The level of interaction between these fillers and non-polar grades of rubber is therefore extremely low. To prepare rubber powder/pellets using these products it is therefore essential to increase the level of mutual interaction. The simplest way to do this is to hydrophobicize the filler surface with the aid of suitable hydrophobicizers. This gives the filler surface a more organic, and therefore less polar, nature, and there is therefore an increase in interactive forces between the filler and the rubber. This measure, which is generally undertaken during preparation of the filler suspension (EP 99 117 178.6; DE 100 56 696.0), permits products to be obtained which comply with specification even when using the abovementioned silicatic and oxidic fillers.
A large of number of patents and publications have appeared (EP 99 117 178.6; DE 100 56 696.0) which include the above-mentioned hydrophobicizing procedure. The hydrophobicizing agent which they describe uses organosilanes which, together with the silicatic or oxidic filler, are suspended in water, in particular with concomitant use of compatabilizers and emulsifiers. After addition of the rubber emulsion and then precipitation via addition of acid, the desired rubber powder forms as a suspension in the water. After mechanical removal of most of the process water, the product is dried thermally to a residual moisture content of less than 3%. The organosilane reacts with the silica at an elevated temperature with formation of siloxane bonds, whereupon large amounts of alcohol, generally ethanol, are liberated (U. Görl, M. Schmitt, paper presented at a DKF conference in Budapest, April 2001).
The use of organosilanes, and particularly the important rubber-technology compounds bis(triethoxysilylpropyl) tetrasulphane and, respectively, disulphane (TESTPT and TESPD) as hydrophobicizing agent for the filler in the process for preparing the rubber powder is certainly logical in this context, since the high-silica-content mixtures increasingly used in car tire tread mixtures since the beginning of the 90's contain relatively large amounts of the abovementioned organosilanes (EP 0 501 227; U.S. Pat. No. 5,227,245; U. LeMaitre, ‘The Tire Rolling Resistance’, AFICEP/DKG Meeting, Mulhouse, France 1993; and G. Agostini, J. Bergh, Th. Materne, paper presented at the Akron Tire Group Technology, Akron, Ohio/USA, Oct. 1994). The use of the silane in the PR preparation process achieves two objectives: firstly that the products are prepared via an increase in the level of filler/rubber interaction (hydrophobicization) and secondly that the filler passed on within the product is presilanized and can be further processed by the customer without the risk of further release of ethanol. This specific property of the product reduces the number of problems, which the user has to solve during the current process, when silane is added directly in the internal mixer. Desirable features would be a marked shortening of the mixing time, the elimination of ethanol releases in the mixing area, and an improvement in vulcanization-related property profile.
The high price of organosilanes, the complex and time-consuming mixing process (A. Hunsche, U. Görl, A. Müller, M. Knaack, Th. Göbel, Kautsch. Gummi Kunstst. 50 (1997) 881; and A. Hunsche, U. Görl, H. G. Koban, Th. Lehmann, Kautsch. Gummi Kunstst. 51 (1998) 525), and the release of large amounts of ethanol (U. Görl, A. Parkhouse, Kautsch. Gummi Kunstst. 52 (1999) 493) mean that organosilanes are used only when it is impossible to achieve the required vulcanizate property profile without the use of these substances. This generally applies to high-performance mixtures, e.g. tire sector applications, specifically those within the tread (EP 0 501 227; U.S. Pat. No. 5,227,245; U. LeMaitre, ‘The Tire Rolling Resistance’, AFICEP/DKG Meeting, Mulhouse, France 1993; and G. Agostini, J. Bergh, Th. Materne, paper presented at the Akron Tire Group Technology, Akron, Ohio/USA, October 1994).
The vast majority of rubber mixtures which contain pale-coloured fillers, often including naturally occurring fillers, use no organosilane or at best only small amounts of organosilane. Examples would be almost the entire shoe sole sector, floor coverings, simple extrusion items, such as profiles, webs, hoses, and items produced by injection moulding, for example gaskets and other mouldings.
A wide variety of grades of pale-coloured silicatic and oxidic fillers is used in the rubber industry. These are the fumed and precipitated silicas and silicates, clays, siliceous chalks, chalks, hydroxides, such as aluminium hydroxides and magnesium hydroxides, and also oxides, such as calcium oxide, zinc oxide, magnesium oxide, and titanium dioxides. As mentioned above, all of these fillers are polar and require hydrophobicization of their surface during the preparation of rubber powder/pellets with use of aqueous emulsions of a non-polar rubber.
If these products are divided according to their surface chemistry, two classes can be distinguished. The surface of clays and siliceous chalks has a considerable number of silanol groups with which organosilanes, for example, can react to form siloxane bonds, hydrophobicizing the filler. At the same time, the reaction product produced from filler and silane represents the first constituent step within the reaction scheme for a bifunctional organosilane in a rubber application. As processing continues to the finished rubber item, specifically during vulcanization, the reaction of the rubber-reactive silane function with the rubber matrix finally takes place with formation of covalent rubber-filler bonds. It is finally these bonds which permit the use of high-activity silicas in high-performance mixtures, in particular in the tire sector.
In contrast, chalks of the hydroxides and oxides mentioned have no silanol groups or, due to their low N2 surface area, only a small number of silanol groups, with which an organosilane could react chemically to form a siloxane bond. There is therefore no formation of rubber-filler bonds which could significantly improve the property profile of the mixtures containing these fillers. Here again, the organosilanes are not used for the reason stated above.
In summary, it may be stated that for mixtures which contain silicatic and/or oxidic fillers and which do not need rubber-filler bonds in order to achieve the property profile required, i.e. simple mixtures, it is desirable to find a low-cost alternative which merely hydrophobicizes the filler and thus permits preparation of rubber powder. These considerations then also make it unimportant whether and to what extent the filler used bears reactive groups on its surface.